Production of the cyanotoxin nodularin—A multifactorial approach

Abstract

Summer blooms in the Baltic Sea are dominated by the diazotrophic cyanobacteria Nodularia spumigena and Aphanizomenon sp. During the blooms, N. spumigena is concentrated to the water surface and exposed to high irradiances of both photosynthetic active radiation (PAR, 400–700 nm) and ultraviolet radiation (UVR, 280–400 nm), in addition, this organism is exposed to seasonal changes in nutrient conditions. N. spumigena produces nodularin, a hepatotoxin lethal to wild and domestic animals. It has been suggested that the accumulation of nodularin within the cell and the release from the cell are affected by different environmental factors. One laboratory experiment and two outdoor experiments were performed to investigate the interaction of two radiation treatments, PAR and PAR + UV-A + UV-B (PAB); three nutrient treatments, nutrient replete (NP), nitrogen limited (–N), and phosphorus limited (–P) and the presence and absence of Aphanizomenon sp. on intracellular as well as extracellular nodularin concentration in N. spumigena. In this study, we hypothesised that the interaction of ambient radiation, nutrient limitation, and the presence or absence of Aphanizomenon sp. would affect the accumulation and release of nodularin. We further hypothesised that the presence of Aphanizomenon sp. would increase the production and release of nodularin and that this increase would have a negative effect on the specific growth rate of this co-existing species. Significant interaction effects were found between the factors investigated. In all three experiments, the lowest intracellular nodularin concentrations were found under phosphorus limitation. The highest intra- and extracellular nodularin concentrations were observed under nitrogen limitation when shielded from UVR. In our opinion, further increase of nitrogen removal in e.g. sewage treatment, should consider a possible increased toxicity of the N. spumigena blooms. The presence of N. spumigena had no significant effect on the specific growth rate of Aphanizomenon sp. under different radiation and nutrient treatments. Thus, we conclude that although nodularin accumulation and release were dependent on different environmental conditions, it did not affect the co-existing species Aphanizomenon sp.